Electric motors increasingly are being used in watercraft because of their greater efficiencies compared to fossil fueled motors. Other advantages prompting use of electric motors include smaller weight, greater reliability and elimination of smoke and fumes when using an electric motor, particularly in combination with all electric battery systems. Typically, a source of electric power, such as a lead acid battery bank or fuel cell, is connected to the electric motor and an electronic valve such as a PMW (pulse modulated width) controller is used to modulate motor power. The motor output is coupled to a propeller either directly through a shaft coupling, or indirectly, through a belt, gear or other mechanism. In practice, a boat operator increases boat speed merely by increasing motor power. To slow down, the operator decreases motor power and/or drives the propeller in the reverse direction. Such simple control systems are used particularly for slow watercraft that are limited to a calculable hull speed above characteristic of a displacement vessel.
Unfortunately, merely increasing power to a propeller does not automatically translate proportionally into increased speed. Accordingly the actual selection of motor power affects overall efficiency, as reviewed here. A spinning propeller will push water with different efficiencies depending on a number of factors. Among these factors are the pitch of the propeller and the speed of the boat. A propeller's pitch usually is expressed as a distance that the propeller edge moves forward in a single revolution. By multiplying the pitch by rpm (revolutions per minute) the distance that an ideal propeller (assuming no friction) can move in one minute is determined.
In the real world a propeller does not move water at the ideal rate but will experience some slip. The slip can be calculated as an “apparent slip” as for example described by David Gerr who points out that water is “a fluid and so a propeller slips or slides a bit as it rotates. It's more exact to view slip as the difference between the distance a boat actually travels through the water—in the time of one complete propeller revolution at her speed through the water, V—and the theoretical distance it would travel if it advanced the full pitch of the propeller. This difference is called ‘apparent slip’ (SlipA) and is expressed as a percent of theoretical propeller advance (pitch times RPM)” (Propeller Handbook p. 48 McGraw-Hill Companies 1989). The term “slip” as used herein means the “apparent slip” defined by Mr. Gerr.
Some propeller slip is necessary for a propeller to accelerate a boat and represents inefficiency in moving the boat. The inefficiency of the drive system at different speeds is not obvious to the user and for the most part is not appreciated unless the user actually see or hears the wasted energy from extreme positive slip as cavitation. Cavitation is an extreme case of propeller slip where so much power is applied to the propeller that the propeller turns rapidly without significantly moving water. During cavitation, much of the propeller energy is lost in the formation of bubbles and some is lost as heat. Cavitation has been a concern of boaters that use internal combustion engines because of the tremendous energy used to achieve and maintain high-speed conditions, and the ease which cavitation can occur under those circumstances.
The cavitation problem has been recognized as a condition of drive system inefficiency and has been recognized for many years. Other inventors have addressed cavitation by discovering new propeller designs and materials (U.S. Pat. Nos. 4,293,280; 4,188,906 5,800,224; 5,711,742; 5,456,200; 5,209,642; 5,083,950; 5,405,276; 2,769,420 and 5,030,149 for example) that withstand the destructive energy from bubble forming and breaking at the propeller surfaces.
More recently, attempts have been made to detect or prevent cavitation of internal engine driven propellers by monitoring water pressure near the propeller and using a water pressure signal to feedback interruption of a spark to the engine. U.S. Pat. No. 5,613,887 describes the placement of one or more pressure sensors that create a mechanical signal that is conducted through a vacuum line and then converted into an electrical signal to indicate pressure. U.S. Pat. No. 5,190,487 uses a bubble detector to detect conditions that precede cavitation. The bubble detector output signal is compared with other signals to output an engine slowdown signal that decreases speed of the internal combustion engine. Another pressure sensor system used to limit combustion of fuel is shown in U.S. Pat. No. 5,833,501, which is designed to prevent cavitation and which emphasizes that this problem is higher “in watercraft having more powerful engines.”
Although each of the patents cited above addresses cavitation in internal combustion engine drive watercraft the general problem of overall efficiency at different propeller speeds and different boat speeds remains under recognized. Furthermore, known systems for alleviating or preventing cavitation in fossil fueled boats are crude and generally merely detect gross cavitation, or caviation-onset conditions but do not specifically detect propeller slippage itself over a wide range. Still further, the systems are designed around fossil fuel burning engines and do not respond instantaneously. Yet further, systems used until now generally rely on pressure sensors or other mechanical detecting devices that are prone to reliability problems and false signal problems inherent to mechanical detecting (pressure, bubbles) systems. Accordingly, these systems do not handle properly the more subtle problems of propeller efficiency, particularly from low power, low speed electric motor driven watercraft. Furthermore, future development of fuel cell powered electric boats of higher speed will present more demands on efficiency and improved control. Thus, much more needs to be done, both for slow moving displacement electric powered watercraft and for faster fuel cell powered watercraft, as well as fossil fuel powered watercraft.